Two-Stage Compressor Control for Liquefied Gas Deep Evacuation
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Solution Overview
Problem
Traditional single stage compressors leave valuable product in supply tanks due to low volumetric efficiency or high discharge temperatures, and operate at a fixed speed that does not maintain a maximum fluid transfer rate, leading to increased costs and inefficiencies in liquefied gas transfer and vapor recovery.
Innovation Solution
A two-stage compressor system with a variable frequency drive and programmable logic controller that adjusts speed based on temperature and pressure changes, allowing for a selectable single or two-stage operation, and includes a liquid trap and multiple valves to optimize fluid transfer and vapor recovery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a single stage compressor is used, then the capacity and efficiency during liquid transfer are maximized, but valuable product remains in the supply tanks due to low volumetric efficiency or high discharge temperatures
Solution Approach 1:
The compression process is divided into two distinct stages: a first compression stage for initial vapor recovery, and a second compression stage for deep evacuation. This segmentation allows the system to first remove the bulk of vapors efficiently, then proceed to extract remaining product that would otherwise be lost, thereby resolving the contradiction between transfer capacity and product recovery.
Solution Approach 2:
The system maintains continuous operation through automated stage transitions. The microprocessor controller automatically switches between first and second compression stages based on real-time pressure and temperature monitoring, ensuring uninterrupted vapor recovery and product extraction without manual intervention, thus maximizing both productivity and substance recovery.
2Productivity
If a fixed operating speed compressor is used, then the system structure is simple, but the fluid transfer rate cannot be maintained at maximum due to changing pressures and temperatures
Solution Approach 1:
The compressor operating speed is made dynamic rather than fixed. The microprocessor controller continuously adjusts the compressor speed based on real-time measurements of pressure and temperature conditions, allowing the system to maintain optimal fluid transfer rate across varying operating conditions while managing complexity through automated control.
Solution Approach 2:
A feedback control system is implemented where pressure and temperature sensors continuously monitor system conditions and feed this information to the microprocessor controller. The controller processes this feedback and automatically adjusts compressor operation accordingly, enabling the system to maintain maximum productivity despite changing external conditions without requiring complex manual adjustments.
3Loss of time
If the compressor operates at high speed to reduce unloading time, then labor and demurrage costs are reduced, but discharge temperatures increase causing product loss
Solution Approach 1:
The unloading process is segmented into two phases: an initial high-speed vapor recovery phase that quickly reduces unloading time and costs, followed by a second stage that operates at optimized speeds to extract remaining product without excessive discharge temperatures. This temporal segmentation resolves the contradiction between speed and product loss.
Solution Approach 2:
The system dynamically changes operating parameters including compressor speed and stage configuration based on real-time conditions. During the first stage, parameters are optimized for rapid vapor removal; during the second stage, parameters are adjusted to prioritize product recovery while managing discharge temperatures, thus resolving the time-loss versus substance-loss contradiction.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system maximizes fluid transfer rate and efficiency by recovering additional product from supply tanks, reducing unloading times and costs by adapting compressor speed to changing conditions.
Implementation Method 1
a two stage compressor... The two stage compressor may be selectably changeable between the single stage mode and the two stage mode
Implementation Method 2
a liquid trap fluidly coupled to the two stage compressor
Data Source
Figure 1
Figure 2
Figure 3A
AI summary
A liquefied gas unloading and deep evacuation system may more quickly, more efficiently and more completely unload liquefied gases from transport tanks, such as rail cars, into stationary storage tanks or into truck tanks. The system may utilize a two stage compressor, an electric motor, a variable frequency drive, a four way valve, a three way valve, a two way valve, a programmable logic controller based control system and pressure and temperature transmitters. The valving enables deep evacuation of the transport or supply tank to more completely empty the transport tank. The programmable logic controller and variable speed drive may be used to variably control the speed of the two stage compressor so that the system may be running as fast as possible during changes in ambient temperature and/or different stages of offloading the liquefied gases without exceeding the compressor's horsepower limit.